When the nuclei of heavy atoms (such as uranium and plutonium) are split energy is released in the form of heat and radiation. Fission can be achieved by bombarding the nucleus with neutrons. A neutron will more easily hit a fissile uranium atom if its speed is slowed down.

Therefore, a moderator is required: a substance that acts like a brake upon the neutrons.

The fissile nucleus will disintegrate into several fission fragments and in turn emit neutrons. These can react in new fission processes, causing a chain reaction.

An enormous amount of radiation and kinetic energy is created when the fission fragments burst apart, and this energy is converted into heat: approximately 24.000.000 kWh (thermal) for 1 kg of uranium 235 (if all of the atoms are split).

By way of comparison, burning 1 kg of coal produces 8 kWh of thermal energy (i.e. 3.000.000 times less).

Industrial fissile fuel, containing only some 3% of actually fissionable material, has about a 100.000 times more energy potential than coal.

After four years, the usable energy in the fuel will be depleted. The spent fuel is then replaced with fresh fuel, then, "reprocessed": this means that the residual uranium and plutonium that can be reused, is recovered. The spent fuel can also be conditioned and stored as high-grade radioactive waste.

The moderator : a brake on neutrons

A neutron will more easily hit a nucleus if it moves at a lower speed.

Hereto, a moderator is used: it acts like a sort of brake on the neutrons. The moderator can be a substance such as water or graphite. PWR reactors use the water of their primary circuit as the moderator.

The neutrons first hit the water molecules and thus lose speed, enabling them to hit a fissile nucleus. Using water as the moderator has a great advantage compared to graphite. First of all, it is unflammable. If the water temperature increase above a certain level, it would simply start to boil - with the corresponding bubbles. Inside these bubbles there would be no water and thus no moderator. Without a moderator the neutrons would not lose speed and would thus not be able to induce fissions: the reaction would eventually stop.

This characteristic is called a negative void coefficient.

Absorbing neutrons

At each fission of a nucleus 2 or 3 neutrons (an average of 2.43) are released. But the reaction required in the reactor is a controlled chain reaction, whereby each fission entails only one new fission, induced by a single neutron. Hence the surplus in neutrons must be eliminated: a part of them will be captured by the wall and internal structures of the reactor vessel: the remainder must be absorbed by means of a controlled and adjustable technique.

By adding boric acid to the primary circuit water, and by inserting control rods in the reactor vessel, neutrons can be absorbed and thus the reaction can be controlled. Both boric acid and the material of the control rods (an indium-cadmium-silver alloy or boric carbide) have the property to absorb neutrons. Depending on the amount of boric acid added to the water, and the number of control rods inserted in the reactor (altogether or partially), the neutron balance will be either negative or positive. When all control rods are at once dropped into the reactor (by gravity) the reaction will stop within 1.3 seconds : this happens during a scram or an emergency shutdown.

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